1. Field of the Invention
The present invention generally relates to a low drop-out voltage regulator with high-performance linear and load regulation and, more particularly, to a low drop-out voltage regulator using a complementary type buffer to overcome poor linear and load regulation and poor stability in a conventional voltage regulator using an n-type buffer or a p-type buffer.
2. Description of the Prior Art
Please refer to FIG. 1, which is a circuit diagram of a conventional low drop-out voltage regulator. The low drop-out voltage regulator comprises: a reference voltage (Vref) circuit, an error amplifier, a power device and a feedback circuit. In applications where a low drop-out voltage regulator is used, a regulation capacitor (Cout) is disposed on the output. Therefore, dominant poles of the low drop-out voltage regulator mostly often appear at the output (Vout). When the output load current increases, the dominant poles move toward higher frequencies to cause poorer reliability of the low drop-out voltage regulator since the output resistance of the power device is in inverse proportion to the output load current.
Please refer to FIG. 2, which is a Bode's plot of a low drop-out voltage regulator. The gain and frequency response relation depends on 1/RC. In FIG. 2, the dominant poles are different for heavy load and light load.
Therefore, frequency compensation of a conventional low drop-out voltage regulator is achieved by adding a voltage buffer between the error amplifier and the power device. With a low output resistance, the voltage buffer moves the pole (for example, the second pole) at the output of the error amplifier outside the frequency band-width. In such a manner, the stability of the low drop-out voltage regulator is assured. Such a conventional voltage buffer uses an n-type or a p-type MOSFET. At steady states, a load current is provided at the output of the low drop-out voltage regulator. Since the feedback control over the power device is not activated yet, the output capacitor has to discharge the load resistor (RL) so that the low drop-out voltage regulator provides the load current. Meanwhile, the output voltage is lowered. As the output voltage is lowered, the error amplifier is activated and the output voltage of the error amplifier is lowered. Therefore, the power device outputs a current to the output capacitor to achieve regulation of output voltage.
Please refer to FIG. 3A and FIG. 3B for circuit diagrams of a conventional low drop-out voltage regulator using a p-type buffer and an n-type buffer, respectively, for frequency compensation. If the voltage buffer for frequency compensation uses a p-channel MOSFET and the low drop-out voltage regulator operates with a heavy load current, the current from the power device decreases because the output voltage of the error amplifier is increased by a voltage of +VSG across the p-type buffer. Therefore, the output voltage of the error amplifier has to be lowered to achieve regulation of output voltage. However, this decreases the loop gain of the low drop-out voltage regulator and leads to poorer load regulation.
On the contrary, if the voltage buffer for frequency compensation uses an n-channel MOSFET, the input voltage of the low drop-out voltage regulator and the low drop-out voltage regulator operates without any load current, the current charging the output capacitor cannot be reduced by the power device and the output voltage of the power device decreases due to leakage current of the power device when there is no load because the output voltage of the error amplifier is decreased by a voltage of −VGS across the n-type buffer. Therefore, the output voltage of the error amplifier has to be enhanced to achieve regulation of output voltage. However, this decreases the loop gain of the low drop-out voltage regulator and leads to poorer load regulation.
Therefore, there exists a need in providing a low drop-out voltage regulator with high-performance linear and load regulation a low drop-out voltage regulator with high-performance linear and load regulation using a complementary type buffer for frequency compensation to overcome poor linear and load regulation and poor stability in a conventional voltage regulator using an n-type buffer or a p-type buffer.